Nitrogen uptake and fixation in the cyanobacterium Cylindrospermopsis raciborskii under different nitrogen conditions

Ammonium and nitrate uptake and N-2-fixation of the heterocystous cyanoprokaryote Cylindrospermopsis raciborskii was examined in continuous cultures under different nitrogen concentrations and dilution rates using the N-15 technique. It was found that at luxury phosphorus supply (5 Mg PO4-P l(-1)) the biomass was similar in all cultures irrespective of the amount and portioning (continuous or pulsed) of available nitrogen forms. The added ammonium and nitrate was fully taken up by C. raciborskii and the remaining nitrogen demand was met by N-2-fixation. Different ammonium concentrations (300, 750, 1500 and 3000 mug N-15 l(-1)) added at the same dilution rate did not affect the growth of C. raciborskii. In the culture supplied with pulsed ammonium, N-2-fixation was detected prior to ammonium addition only. After the ammonium pulse, the N-2-fixation continued for a while then decreased and stopped. In addition, the inflowing ammonium was fully taken up by the organism. The rate of nitrogen fixation reached its original level after 8-24 hours, depending on the dilution rate. It can be suggested that the nitrogen fixation system stopped and was then activated again depending on the nitrogen content of the cells.